This invention relates to a super fine thermoplastic fiber coating or layer on a strip or sheet base member and, more particularly, to a polypropylene fiber coated polypropylene dielectric strip for use as a capacitor dielectric in a dielectric liquid impregnated capacitor.
Polypropylene film as a capacitor dielectric has become a prominent and outstanding dielectric for liquid impregnated capacitors. In U.S. Pat. No. 3,363,156, Cox, assigned to the same assignee as the present invention, there are disclosed numerous capacitor embodiments employing the combination of polypropylene film and chlorinated diphenyl liquid impregnant. The teachings of this patent are incorporated by reference in the present application.
The noted Cox patent describes a kind of impregnation of polypropylene film as "essentially complete impregnation" whereby the liquid impregnant passes through or permeates the molecular structure of the polypropylene film. In a capacitor roll section which is wound from a composite of alternate foil and film strips, the kind of impregnation referred to is most important but difficult to obtain because the liquid impregnant must proceed from the roll edge into the center parts of the roll. Polypropylene films tend to cling tightly to each other and to electrode foils, so that it is extremely difficult to provide essentially complete impregnation of all voids, interstices and spaces within the roll as well as within the materials.
In order to more expeditiously provide essentially complete impregnation, it has been the practice to utilize paper strips adjacent the film strips in a roll so that the paper may act as wicking to pass the liquid impregnant to the center parts of the roll. Paper strips are undesirable in that they represent an undesirable different dielectric in an otherwise all-film dielectric capacitor, and they reduce the volumetric efficiency and increase the dissipation factor of an otherwise all-film dielectric capacitor. The paper also represents a limitation on the kind of impregnant which may be employed. Any unimpregnated areas are sources of deleterious corona discharge even though they may later become impregnated by aging. Some areas become effectively blocked from being impregnated for a considerable period of time because of material nonuniformities, the presence of other materials, and other environmental causes.
Accordingly, it is known in the art that adjacent sticky surfaces of polypropylene to polypropylene, or polypropylene to foil may be modified to permit easier penetration of the impregnant. Among these modifications are included roughening of adjacent surfaces of abrasion and mechanical working, or coating of the surfaces with fine particle material such as talc and aluminum oxide. There are distinct disadvantages to these processes ranging from adversely affecting the physical characteristics of the dielectric, adding foreign matter to the capacitor, and particle control and uniformity.
It has been discovered that these and other disadvantages may be overcome by coating one or more adjacent dielectric surfaces with a light coating of super fine fibrous thermoplastic material. This fibrous material maintains sufficient uniform separation of adjacent surfaces to facilitate the entry and continued permeation of the impregnant.
Accordingly, it is an object of this invention to provide an improved super fine thermoplastic fiber coated base member as improved impregnation means for electrical capacitors.
It is another object of this invention to provide a super fine thermoplastic fiber coated dielectric surface in electrical capacitors.
It is a specific object of this invention to provide a polypropylene fiber coated polypropylene dielectric for electrical capacitors.